The invention relates to power supplies, and more particularly to switching power supplies.
Switching power supplies are commonly used in situations where size and weight are important considerations. A good example is aviation applications where excessive weight and size of avionics devices limits payloads and the amount of avionics capability which can be installed in an aircraft.
Switching power supplies operate by switching a DC voltage, typically with power field-effect-transistors (FETs) to synthesize an AC waveform. The AC waveform is rectified and filtered to produce a new DC voltage level. By changing either the pulse width or frequency of the switching waveform, the average value of the AC waveform and thus the output DC voltage level can be varied. By feeding back the output voltage level to a control circuit, the switching waveform can be automatically adjusted to provide a desired output voltage level.
One factor in the size of a switching power supply is the switching frequency. It is desirable to switch at higher frequencies for several reasons. First, higher frequencies permit use of smaller transformers and filter capacitors. Second, higher frequencies permit use of resonant switching which requires an FM drive signal. FM switching power supplies require a wide frequency deviation in order to achieve the desired regulation range. The ability to switch at higher frequencies is necessary to cover the regulation range using frequency modulation control.
Unfortunately, power supply switching frequencies are now typically around 100 KHz, and they reach an upper limit of usability around 300 KHz. The limitation is caused by the fact that FET gates are actually fixed-value capacitors. As the frequency increases, the FET gate impedance decreases according to the well-known formula: ##EQU1## The Miller effect through the gate/drain capacitance lowers the impedance even more as the FET switches "on". In order to provide the required switching level and speed to the FET gate, the current of the drive signal must increase as frequency increases. An undesirable side effect of increasing the drive current is heating in the bipolar transistors which are normally used in the driver stage. This heating is caused by the inherent 0.7 volt drop in the base-emitter junction of the transistor. Excessive drive power levels obviously lead to failure of drive transistors. Similar problems are associated with field-effect-transistors used as drivers.
It is therefore an object of the present invention to increase the frequency at which FETs can be efficiently and practically switched.
It is another object of the present invention to permit construction of smaller and lighter switching power supplies.
It is a further object of the present invention to permit more efficient switching waveforms to be used in switching power supplies.
It is yet another object of the present invention to alleviate heat failure in bipolar and other transistors used to drive power switching devices in switching power supplies.
Still other objects will become apparent in the following summary and description of a preferred embodiment of the invention.